Field of the Invention
This invention relates to antenna systems integrated into wireless mobile devices, and in particular, to antenna systems capable of dynamically changing radiation modes which result in variable radiation patterns.
Description of the Related Art
There is a current need for improved connectivity at cellular and data transmission bands for mobile devices to accommodate the increasing demand for data rates for mobile wireless systems. Improved antenna performance, such as increased efficiency or optimization of the radiation pattern, will translate into increased data rates. A method for increasing antenna system performance in wireless devices is to increase antenna volume; unfortunately, the trend in mobile devices is to decrease overall product size along with increasing the number of functions required to be integrated into the platform. Improving the directive properties or beam pointing attributes of an antenna will result in improved communication link performance due to the increase in antenna gain in the desired direction of propagation.
To complicate the antenna design process in mobile devices, antenna performance needs to be optimized and characterized for several use cases such as against the user's head, in hand, and against the body. These multiple use cases result in a variation in antenna total efficiency as well as a variation in radiation pattern characteristics (pattern shape and polarization properties).
Beam steering techniques have been implemented for many years on the base station side of the cellular communication link, with the beam steering providing improvements in radiated field strength on the transmit function and RSSI on the receive function. Inherent improvements in interference can also realized by beam forming and steering techniques by better spatial distribution of the radiated signal. Beam steering on the base station typically takes the form of a traditional array where multiple antenna elements are connected to a feed network and amplitude and phase at each element is controlled to provide the preferred antenna beamwidth and directivity. To date, beam steering techniques have not been implemented in mobile devices due to limitations in volume available for multiple antenna elements that can be arrayed to provide a more directive radiation pattern. Another issue restricting the implementation of beam steering in mobile devices is the potential for a two element array providing little or no benefit when one of the two elements is covered by a user's hand or otherwise degraded by body loading.
A beam steering system which utilizes a single antenna with a single feed port is capable of generating multiple radiation patterns is described in U.S. Pat. No. 7,911,402, entitled “ANTENNA AND METHOD FOR STEERING ANTENNA BEAM DIRECTION,” issued on Mar. 22, 2011, the contents of which are hereby incorporated by reference. This technique lends itself well to small mobile devices which are volume constrained. This technique relies on an offset parasitic element to alter the current distribution on the radiating element and a second parasitic element more closely coupled to the radiator to adjust the frequency response of the antenna. This beam steering technique is implemented on a planar structure which is elevated above a ground plane, making this a good option for an internal antenna in a cell phone. Implementing this approach on both the main and secondary antennas in a MIMO antenna system provides multiple radiation patterns to select from and also provides for multiple radiation patterns from the single antenna that is not affected during a situation where hand loading or body loading degrades one of the two antennas in a mobile wireless device. This antenna topology does not lend itself to implementation in handsets which have a metal ring wrapped around the circumference of the device.
Another antenna technique involves the use of coupling gaps in a ring type structure to allow for use of the ring structure encompassing a mobile device to be used as an antenna, and is further described in U.S. application Ser. No. 13/609,138, entitled “ACTIVE ANTENNA STRUCTURE MAXIMIZING APERTURE AND ANCHORING RF BEHAVIOR” filed on Sep. 10, 2012, the contents of which are hereby incorporated by reference. The coupling regions designed into the gaps assist in minimizing the de-tuning effects in the ring to hand or body loading. The overall structure can be considered as a capacitively loaded inductive loop. The capacitance is formed by the coupling between the two parallel conductors with the inductive loop formed by connecting the second element to ground. The length of the overlap region between the two conductors along with the separation between conductors is used to adjust the resonant frequency of the antenna. A wider bandwidth can be obtained by increasing the separation between the conductors, with an increase in overlap region used to compensate for the frequency shift that results from the increased separation.
An advantage of this type of antenna structure is the method in which the antenna is fed or excited. The impedance matching section is almost independent from the resonant portion of the antenna. This leaves great flexibility for reduced space integration. At resonance a cylindrical current going back and forth around the loop is formed. This generates a magnetic field along the axis of the loop which is the main mechanism of radiation. The electrical field remains highly confined between the two elements. This reduces the interaction with surrounding metallic objects and is essential in obtaining high isolation.